Microelectromechanical systems (MEMS) are being utilized in a variety of micro-sensor applications, with specific applications including accelerometers having a component that is moveable with respect to the main body or substrate of the device when subjected to acceleration, whereby a resulting change in a capacitive gap is used to generate an electrical output that can be correlated to the magnitude and/or direction of the acceleration. The MEMS device should be fabricated to form a functional element in a semiconductor wafer having an overall structure of small dimensions.
MEMS fabrication processes commonly use a release technique to free the moveable component of the MEMS structure from the substrate or underlying films to allow the structure to freely move. The most common release method is to remove an oxide film from under a poly-silicon or silicon device layer using a wet etch technique. The use of a wet etch leads to stiction, which occurs when the moveable component contacts another surface of the device and becomes permanently or temporarily bonded to the other surface.
Attempts to overcome the stiction problem associated with the use of wet etching have involved methods for removing the liquid etchant by alcohol displacement and subsequent evaporation. These techniques require additional steps, time and equipment, and therefore add significantly to the overall manufacturing cost.
Recently, MEMS devices have been produced using deep reactive ion etching (DRIE) of silicon-on-insulator (SOI) wafers. This technique is used to remove material from the silicon device layer until the buried oxide layer is reached. The buried oxide layer is then removed with a wet etch technique or with vapor phase hydrofluoric acid (HF) etching. The wet etch technique results in stiction. The HF vapor process eliminates stiction but attacks most common semiconductor films.